The present invention relates generally to the field of medical imaging. More particularly, the invention relates to magnetic resonance imaging and to the calculation of receive coil sensitivities.
In the field of medical magnetic resonance imaging, the patient is placed within a spatially uniform magnetic field (B0). The individual magnetic moments of the spins in the tissue generally align with this polarizing field, precessing about it in a random manner dependent on their characteristic Larmor frequency. Gradient magnetic fields are employed which, within the three-dimensional imaging volume, impute characteristic magnetic field differences with the B0 field. The patient tissue is then subjected to an rf excitation pulse (B1) which is near the Larmor frequency and which is perpendicular to the B0 field, to rotate or xe2x80x9ctipxe2x80x9d the spins into the plane transverse to the B0 field to produce a net transverse magnetic moment. Once the rf excitation pulse is terminated, the spins realign with the B0 field and, in doing so, emit a magnetic resonance signal, which may be localized by means of the gradient magnetic fields, and which can be detected and processed via Fourier transformation to form an image. The rf signals are typically applied by a xe2x80x9ctransmitxe2x80x9d coil and resulting signals are detected by a xe2x80x9creceivexe2x80x9d coil. In certain systems these functions are combined in a single coil or coil set. The magnetic resonance signals are acquired as a voltage induced in the receive coils within the imaging system.
The duration of the scan time is determined, in part, by the number of phase encoding steps performed, which is itself dependent upon the desired image size and image resolution. To produce diagnostic quality images, magnetic resonance imaging techniques typically require many minutes to acquire the imaging data. Reduction of scan time is therefore a desirable goal in order to improve patient comfort, increase patient throughput, and reduce image artifacts resulting from inadvertent patient movement.
One technique for reducing image acquisition time is to reduce the number of phase-encode steps, such as by keeping the same phase-encode gradients but only collecting every other column of data, thereby halving the collection time. The spacing of the data points in the phase-encode direction is thus doubled, so that the field of view in the corresponding image domain is effectively halved. Unfortunately, resonance signals from outside the reduced field of view are still detected by the receive coils and are xe2x80x9cfoldedxe2x80x9d back upon the image, or aliased, such that an aliased pixel represents intensity data for more than one point within the imaging volume. Such aliasing is thus undesirable in that it adversely affects image quality.
One technique for canceling these unwanted signals is commonly referred to as sensitivity encoding, or SENSE, which utilizes the spatial sensitivity profile in a multiple receive coil system to determine signal position information within the region of interest. The SENSE reconstruction of these multiple receive coil signals enables aliasing to be reduced, i.e., the aliased image is xe2x80x9cunfolded,xe2x80x9d using the respective sensitivity information for each receive coil. In particular, the sensitivity information for each coil is a complex function which describes the coil""s response to resonance signals originating from different points in the imaging volume. Use of this information allows the removal of aliasing effects. In this manner, a full image may be produced though the acquisition time is only that required to obtain a reduced field of view image, i.e., the acquisition time may be halved.
To obtain the necessary sensitivity information, a calibration scan is performed in addition to the diagnostic scan. Because the ultimate goal is to reduce the system scan time, this calibration scan should be as rapid as possible. The calibration scan time may be shortened by reducing the spatial resolution of the scan. Unfortunately, the reduced resolution produces an inaccurate assessment of coil sensitivities near the edges of the object which can prevent the removal of some aliasing artifacts. Another problem is that inadvertent patient motion (e.g., breathing) or a change in position (e.g., no or relatively small breathholding volume during calibration and larger breathholding volume during imaging) between the diagnostic and calibration scans may result in some areas which are not imaged during the calibration scan but which are imaged during the diagnostic scan. Because of the absence of a signal in these areas during the calibration scan, no coil sensitivity information is available to remove the aliasing artifacts from these areas.
There is a need, therefore, for an improved technique for obtaining coil sensitivity information during calibration scans when the SENSE technique is employed. To address the drawbacks in existing systems, there is a particular need for a technique which can be employed in a straightforward manner to allow the accurate estimation of coil sensitivities near object edges and in pixels with no measurable sensitivity, i.e., xe2x80x9cemptyxe2x80x9d pixels. The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.
The invention provides a novel technique for constructing a coil sensitivity matrix for the receive coils of a magnetic resonance imaging system. The technique utilizes a derived sensitivity function to approximate coil sensitivity near the edges of an object. In this manner, a modified receive coil sensitivity matrix may be determined which allows a diagnostic image to be corrected for aliasing artifacts.
The determination of the sensitivity matrix is performed by performing a calibration scan at reduced resolution. The data from the scan is processed to form a calibration image. Within each column of the image, the edges of the imaged object are located and a sensitivity function for each edge is derived based upon a number of pixels located inward of the edge. The sensitivity function typically comprises a linear extrapolation though other statistical fit models may be used. Based upon this mathematical model, an estimation can be made of the actual sensitivity of those pixels near the edge of the object. In this manner, the sensitivity values of the pixels inward of the object edge, which are subject to inaccuracy due to their proximity to the edge, may be replaced with the respective calculated sensitivity values. In addition, a calculated sensitivity may be assigned to pixels outward from the object edge for which no sensitivity may be measured due to lack of a resonance signal.
In accordance with one aspect of the technique, a method is provided for estimating coil sensitivities in a magnetic imaging system. Within a magnetic resonance image, an edge pixel is located within the columns or rows of an image. A sensitivity function is calculated which describes coil sensitivity for that edge pixel. The sensitivity function is calculated using two or more fitting pixels inward of the edge pixel.
In accordance with another aspect of the technique, a method is provided for generating an enhanced sensitivity matrix for an object. An initial calibration image of the object is first obtained. Object edges within the columns or rows of the calibration image are then located which comprise object edge pixels. A sensitivity function for each object edge is then calculated using two or more fitting pixels located inward of the object edge pixels. Respective sensitivity values are then derived from the sensitivity function and assigned to each of one or more pixels located outward from the object edge pixels.
In accordance with another aspect of the technique, magnetic resonance imaging system capable of estimating coil sensitivity is provided. The magnetic resonance imaging system comprises a magnetic resonance scanner capable of generating a calibration image. In addition, the system comprises an analysis circuit capable of receiving the calibration image from the scanner. The analysis circuit processes the image by locating edge pixels within columns or rows of the image and by calculating a sensitivity function from two or more fitting pixels disposed inward of the edge pixels. The sensitivity function describes coil sensitivity near the edge pixels.
In accordance with another aspect of the technique, a magnetic resonance imaging system is provided capable of generating an enhanced sensitivity matrix for a subject. The system comprises a magnetic resonance scanner capable of generating a calibration image. In addition, the system comprises an analysis circuit capable of receiving the calibration image. The analysis circuit calculates a sensitivity function from two or more fitting pixels disposed inward of edge pixels. This sensitivity function describes coil sensitivity near the edge pixels. The analysis circuit further assigns a respective calculated sensitivity derived from the sensitivity function to each of one or more pixels disposed outward from the edge pixels.
In accordance with another aspect of the technique, a magnetic resonance imaging system is provided capable of generating an optimized image of a subject. The system comprises a magnetic resonance scanner capable of generating a diagnostic image and a calibration image and an analysis circuit capable of receiving the diagnostic image and the calibration image. The analysis circuit comprises a means for generating an enhanced sensitivity matrix using the calibration image. The analysis circuit optimizes the diagnostic image using the enhanced sensitivity matrix to generate an optimized diagnostic image. The system further comprises a display circuit capable of receiving the optimized diagnostic image and transmitting the optimized diagnostic image to a suitable display device.